Gassantoda Castle, Mountain castle in Yasugi, Japan.

Gassantoda is a ruined hilltop castle in Yasugi, Shimane Prefecture, that spreads across the ridges and valleys of Mount Gassan, standing 180 meters above the Hirose River. The site consists of several fortified levels connected by stone steps and pathways that offer wide views over the surrounding landscape.

The castle was held by the Amago clan from 1396, and under Amago Tsunehisa it became their base for expanding across Izumo. Amago Haruhisa later controlled eight provinces of the San'in and Sanyo regions. In 1566 the starving garrison surrendered to the Mori, and the castle was abandoned in 1611 when Horio Tadaharu moved to Matsue Castle.

The castle's name combines Mount Gassan with Tomida, the district of Yasugi where it stands. At the foot of the Sugatani entrance sits Jōan-ji, a Rinzai temple whose grounds are said to cover the site of the Amago clan's residential mansion, the sato-yashiki. The temple keeps a painted castle map by Horio Tomonari's court painter Horie Yūsei and a picture scroll of the Amago Ten Braves.

The ruins can be reached by taking an hour-long bus from Yasugi Station to the Gassan-Iriguchi stop, followed by a 40-minute uphill walk. The path leads over narrow trails and uneven ground, so sturdy footwear and reasonable fitness are necessary.

Amago Tsunehisa retook the castle in 1486 by surprise, attacking from the Karamete gate after slipping past the defenders; the sources record no festival-dancer disguise. The gate's stone walls still stand on the slope below the palace terrace.